1. Field of the Invention:
This invention relates to a saw-tooth wave generating circuit and more particularly to an improvement on a saw-tooth wave generating circuit generally used, for example, in a frequency-to-voltage converting circuit for servo circuits of varied kinds.
2. Description of the Prior Art
FIG. 1A shows a saw-tooth wave generator of a frequency-to-voltage converter used in various kinds of servo circuits. The illustrated circuit includes a constant voltage source KV1 of a suitable value; a charging current limiting resistor R1; NPN type transistors Tr1 and Tr2 having a common base; a power supply Vcc; PNP type transistors Tr3 and Tr4 having a common base; a charging capacitor C; and an NPN type transistor Tr5 for discharging the electric charge of the capacitor C. This saw-tooth wave generator of the prior art thus includes components which form a constant current charging circuit composed of the transistors Tr1-Tr4 and the capacitor C and form a discharge circuit which consists of the transistor Tr5. In the charging circuit, the capacitor C accumulates an electric charge which can be expressed as: Charging current ##EQU1## wherein VBE (Tr1) represents a voltage between the base and emitter of the transistor Tr1. The electric charge is instantaneously discharged through the transistor Tr5 by a pulse current coming through a terminal C-IN. The output thus produced has a saw-tooth wave form as shown in FIG. 1B. However, the above-stated voltage VBE varies with temperature and the charging current I also varies. This causes some change in the inclination of the saw-tooth wave produced, as indicated by the broken lines in FIG. 1B. The peak voltage value of the saw-tooth wave outputs thus has a temperature dependency. This has been a drawback of prior art circuit arrangements.
Because the current amplification factor of the complementary transistors Tr3 and Tr4 depends on the power supply voltage Vcc, the peak voltage value of the saw-tooth wave output has a power supply voltage dependency. This represents another drawback. This is especially serious for a long period saw-tooth wave and has presented a grave problem in saw-tooth wave generating circuits used for frequency-to-voltage converting circuits in servo circuits and the like.